1. Field of the Invention
The present invention relates to a pressure sensitive adhesive composition, and a pressure sensitive adhesive sheet. More particularly, the present invention relates to a pressure sensitive adhesive composition meeting a social demand, that is, a pressure sensitive adhesive composition not containing toluene or a halogen compound.
2. Description of the Related Art
In a field of organic solvent-type pressure sensitive adhesive compositions, a compound having isocyanate groups is conventionally used as a curing agent for copolymers prepared by copolymerizing radical-polymerizable monomers. In this case, the polymers as a base component are often polymerized in the presence of toluene. Toluene acts not only as a solvent for the monomers used, and the resulting polymers, but also as a chain transfer agent in the polymerization. After the polymerization, in many cases toluene is also used as a diluent for adjusting the viscosity. However, from an environmental standpoint a pressure sensitive adhesive composition not containing toluene is now desired.
Thus, the present inventors attempted to obtain a pressure sensitive adhesive composition by using copolymers prepared by polymerizing radical-polymerizable monomers in the presence of an organic solvent (such as ethyl acetate) other than toluene. However, a weight average molecular weight or a number average molecular weight of the resulting copolymer prepared by polymerizing an organic solvent other than toluene is increased in comparison with those of the copolymer prepared by using toluene. As a result, in comparison with the conventional pressure sensitive adhesive composition containing the polymers polymerized in the presence of toluene, the pressure sensitive adhesive composition containing the polymers polymerized in the organic solvent other than toluene has a greater viscosity, under the condition that the solid content is same as that of the conventional composition, and thus the coating property is remarkably lowered. The coating property can be improved by lowering the solid content during the polymerizing step or by dilution after polymerization. If the coating property is enhanced as above, however, only an adhesive sheet having a lowered adhesive strength or tackiness can be obtained.
Generally, an isocyanate group exhibits a high reactivity with a compound having hydroxyl or amino groups. Therefore, an isocyanate compound is often used as a curing agent for a base component, in a composition containing a compound with hydroxyl or amino groups as the base component.
Further, in the field of organic solvent-type pressure sensitive adhesive compositions containing an isocyanate compound, not only the compound with hydroxyl or amino groups, but also a compound with carboxyl groups is conventionally used as the base component of the pressure sensitive adhesive composition. An isocyanate compound is also often used as a curing agent for the base component having carboxyl groups.
An isocyanate compound used as the curing agent is highly reactive. Therefore, after it is mixed with the base component, it is easily reacted with water contained in the organic solvent or air. When the isocyanate compound is reacted with water contained in the organic solvent or air, an insoluble product is formed, and thus a pressure sensitive adhesive composition becomes white and turbid. The present inventors found that such a white and turbid pressure sensitive adhesive composition resulted in only an adhesive sheet with poor adhesive properties, and that this tendency was pronounced when the base component having carboxyl groups without a hydroxyl or amino groups was used.
The time wherein the coating properties or properties of an adhered article or an adhesive sheet are not affected after a curing agent is mixed with a base component is generally called a “pot life”. It is very important to make the pot life as long as possible. Although a required or allowable pot life is different among companies manufacturing an adhered article or an adhesive sheet which actually carry out coating and drying processes, it is generally required that the appearance and properties of a pressure sensitive adhesive composition and properties of an adhered article or an adhesive sheet do not change for at least two hours after a curing agent is added.
Therefore, in order to lengthen the pot life, a halogen compound is generally added to a pressure sensitive adhesive composition as a moisture trapping agent for water contained in an organic solvent or moisture in air. However, from an environmental standpoint a pressure sensitive adhesive composition without a halogen compound is now desired. That is, a pressure sensitive adhesive composition which does not contain a halogen compound, but has a pot life the same as or longer than that of a pressure sensitive adhesive composition containing a halogen compound is desired.
An adhesive sheet carrying a pressure sensitive adhesive composition is widely used in various applications. For example, in the case of a pressure sensitive adhesive composition for use as a building material, a part of household electrical equipment, wallpaper, or the like, the pressure sensitive adhesive composition remains on the adhered articles in a room of a house. In these cases, some components contained in the pressure sensitive adhesive composition may vaporize and float in the air of the room. The substance which may vaporize includes formaldehyde, toluene, or the like, and such chemical substances are regarded as a cause of the so-called “sick-house” syndrome. The “sick-house” syndrome can be prevented if a pressure sensitive adhesive composition not containing chemical substances which cause such a syndrome is used for the adhered article placed in a room.
Even if the substances which may cause “sick-house” syndrome are not used, however, formaldehyde is contained in air, and air may possibly be trapped in the pressure sensitive adhesive composition. Further, formaldehyde may be generated by heating. Thus, it is difficult to control formaldehyde.
Therefore, from an environmental standpoint an adhesive sheet prepared from a pressure sensitive adhesive composition not containing formaldehyde, and able to inhibit the generation of formaldehyde is desired.
As a method of decreasing formaldehyde, Japanese Unexamined Patent Publication No. 2-232279 discloses the use of a formaldehyde trapping agent selected from a group consisting of urea, acetyl acetone, and a combination of urea and a promoting agent selected from a group consisting of glyoxal and acetyl acetone, and Japanese Unexamined Patent Publication No. 11-197501 discloses the use of an aldehyde adsorbent agent comprising an active energy ray-curable resin containing an acetoacetyl group. Further, it is known to use urea, thiourea, ethylene urea, dicyanediamide, glyoxalmonourethane, amines, or amides as a trapping agent.
However, ureas are not soluble in a solvent-type pressure sensitive adhesive composition. Further, dicyanediamides or amines easily react with a base component of the solvent-type pressure sensitive adhesive composition, and thus, the pot life of the pressure sensitive adhesive composition is shortened when dicyanediamides or amines are used.